1. Field of the Invention
The invention generally relates to a circumferential seal assembly with bifurcated hydrodynamic flow for use within a gas turbine engine and more particularly is concerned, for example, with a pair of annular seal rings separately disposed within an annular seal housing about a rotatable runner attached to a shaft, wherein the runner further includes a plurality of stepped hydrodynamic grooves which separate and direct flow onto each annular seal ring to form a pair of thin-film layers sealing one compartment from another compartment.
2. Background
Turbine engines typically include a housing with a plurality of compartments therein and a rotatable shaft that passes through adjoining compartments separately including a gas and a lubricant. Leakage of a lubricant from one compartment into another compartment containing a gas could adversely affect performance and function of a gas turbine. Leakage of a gas from one compartment into another compartment containing a lubricant is likewise detrimental. As such, adjoining compartments must be isolated from one another by means of a sealing system that prevents one fluid, either a lubricant or a gas, from migrating along a rotatable shaft and entering a compartment so as to mix with another fluid therein.
In the case of an aircraft engine, leakage of a lubricant or a gas across a seal into a neighboring compartment may cause oil coking or an engine fire. Oil coke is a byproduct formed when an oil lubricant and a gas mix at a temperature that chemically alters the oil. Oil coke can foul sealing surfaces thereby degrading bearing lubrication and impairing the integrity of a seal. It is important in similar applications, not just aircraft engines, that a lubricant be isolated within a lubricant sump and that a seal around a rotating shaft not allow a lubricant to escape the sump or a hot gas to enter the sump. Many applications will include either a circumferential seal or a face seal to prevent mixing of an oil lubricant and a hot gas; however, circumferential shaft seals are the most widely used under the above-noted conditions.
Various circumferential seal systems are employed within the art to form a seal between a compartment containing a gas at a high pressure and a compartment containing an oil lubricant at a low pressure. Sealing systems typically include grooves disposed along an inner annular surface of a seal ring to form a thin-film layer between the seal ring and a shaft.
Presently known circumferential seal designs are problematic when both adjoining compartments are at a low pressure. The absence of a significant pressure differential between compartments does not permit formation of a thin-film layer adequately capable of preventing migration of a fluid along the interface between a seal ring and a shaft.
Presently known circumferential seal designs are further problematic when used in conjunction with a translatable runner. The temperatures and/or mechanical loads within a turbine engine often cause a runner, and sealing surface thereon, to translation along the axial dimension of an engine. The result is a sealing interface that is difficult to optimize over the operational range of a turbine engine.
Accordingly, what is required is a circumferential seal assembly interposed between a pair of compartments that minimizes degradation to and/or failure of a seal between a rotatable runner and a pair of seal elements.